Thermal sensor with thermal barrier

ABSTRACT

The invention provides a sensor element formed in a first substrate and having a thermal barrier disposed between the sensor element and a heat source provided elsewhere on the first substrate. The thermal barrier includes at least one pair of trenches formed within the first substrate, individual trenches of the pair being separated by a cavity.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/045,910, filed Jan. 26, 2005 and titled “Sensor,” and is acontinuation of International Application PCT/EP/050174, filed Jan. 12,2006, the latter claiming Paris Convention priority to said U.S. patentapplication Ser. No. 11/045,910, which applications are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to sensors and in particular to a thermalsensor formed in a semiconductor substrate. The invention particularlyrelates to such a thermal sensor with a thermal barrier formed betweenthe sensor and a heat source on the substrate such that the output ofthe thermal sensor can be insulated from any spurious contribution fromheat sources co-located on the substrate.

BACKGROUND

Sensors are well known in the art. When formed in a semiconductormaterial such as silicon or germanium such sensors may be provided asmechanical structures, for example as a MEMS arrangement, orelectromagnetic (EM) radiation sensors such as infra-red (IR) sensors.By using materials such as silicon it is possible to form the sensor inone or more layers of the wafer from etching and other semiconductorprocessing techniques so as to result in a desired configuration. Due tothe delicate nature of the sensors and their sensitivity to thesurrounding environment it is known to provide a protective cap over thesensor, the cap serving to isolate the environment of the sensor fromthe ambient environment where the sensor is operable.

Within the area of EM sensors there is a specific need for sensors thatcan be provided in a packaged form.

SUMMARY

These and other problems are addressed by a thermal sensor in accordancewith the teaching of the invention. Such a sensor includes a thermalbarrier located in the substrate between the sensor elements and anysource of spurious heat that may be co-located on the same substrate.

There are many aspects to the sensors described below, the fabricationand use of some, but not all, of those aspects will be summarized here.

According to a first aspect, there is provided a thermal sensor having afirst radiation sensing element provided in a first substrate, the firstradiation sensing element providing an output indicative of theintensity of radiation incident on the sensing element. The sensorincludes a thermal barrier located between the first radiation sensingelement and a heat source which is co-located on the same firstsubstrate. The thermal barrier includes at least one set of trenches,each set having at least a first and second trench, the trenches of eachset being separated from one another by an evacuated cavity. The thermalbarrier is physically separated from each of the first radiation sensingelement and the heat source so as to physically isolate the firstradiation sensing element from the heat source thereby preventing heatsource influence from the first radiation sensing element. The cavitymay be populated with a gas other than air, and it may be dimensioned toextend below the first radiation sending element.

According to another aspect, a sensor array is shown, including aplurality of sensors. Each of the sensors may have an active sensorelement and a reference sensor element. The active sensor element isformed in a first substrate and has an optical element formed in asecond substrate. The first and second substrates are configuredrelative to one another such that the second substrate forms a cap overthe sensor element. The optical element is configured to guide incidentradiation on the cap to the sensing element. The reference sensorelement is also formed in a first substrate and serves to shield thereference sensor element from at least a portion of incident radiationon the cap. The substrate includes at least one additional heat sourcewhich is thermally from the plurality of sensors by providing a thermalbarrier extending downwardly from an upper surface of the substrate andinto the substrate and located between the heat source and the pluralityof sensors. The thermal barrier includes at least one set of trenches,each set having at least a first and second trench, the trenches beingseparated from one another by an evacuated cavity, with the thermalbarrier providing a discontinuation in the substrate material betweenthe plurality of sensors and the heat source.

According to yet another aspect, a discriminatory sensor is configuredto provide a signal on sensing a heat-emitting body, the sensorincluding a first sensor element configured to provide a signal onsensing the body a first distance from the sensor and a second sensorelement configured to provide a signal on sensing the object a seconddistance from the sensor. Each of the first and second sensor elementsincludes at least one sensing element formed in a first substrate and atleast one optical element formed in a second substrate. The first andsecond substrates are configured relative to one another such that thesecond substrate forms a cap over the at least one sensing element. Theat least one optical element is configured to guide incident radiationon the cap to the at least one sensor element. The sensor furtherincludes a thermal barrier located between the sensor elements andsubstrate-provided heat sources co-located on the same first substrate,such barrier including at least one set of trenches. Each set oftrenches has at least a first and second trench, the trenches beingseparated from one another by an evacuated cavity and the thermalbarrier being physically separated from the at least one sensor elementand the heat sources.

Another aspect is a gas analyzer including at least one sensor elementformed in a first substrate and at least one optical element formed in asecond substrate, the first and second substrates being configuredrelative to one another such that the second substrate forms a cap overthe at least one sensor element. The at least one optical element isconfigured to guide incident radiation on the cap to the at least onesensor element, the incident radiation guided having a wavelengthindicative of the presence of a specific gas. The gas analyzer includesat least one reference sensor element formed in the first substrate andhaving a cap for the at least one reference sensor element formed in asecond substrate, the cap serving to shield the reference sensor elementfrom the incident radiation on the cap such that the reference sensorelement provides an output independent of the intensity of the incidentradiation. The analyzer is thermally insulated from heat sourcesco-located on the first substrate by provision of a thermal barrierbetween the analyzer and the heat sources, the thermal barrier includingat least one pair of trenches extending vertically downwardly into thefirst substrate from an upper surface thereof, the trenches beingseparated from one another by an evacuated cavity, such that the thermalbarrier provides a discontinuity in the substrate material between thereference sensor element and the heat sources.

A still further aspect is a method of forming a sensor. At least onesensor element and at least one reference sensor element are formed in afirst substrate. A thermal barrier is formed around the sensor elements,the thermal barrier including a pair of trenches extending downwardlyinto the substrate from an upper surface thereof and separated from oneanother by an evacuated cavity, such trenches serving to thermallyinsulate the sensor elements from other heat sources co-located on thefirst substrate by providing a discontinuity in the substrate materialbetween the sensor elements and the other heat sources. An opticalelement and a shielding cap are formed in a second substrate and the twosubstrates are bonded together such that the second substrate providesthe optical element over the sensor element. The optical element isconfigured to guide incident radiation onto the sensor element. Thesecond substrate provides a shielding cap over the reference sensor, thecap serving to prevent a transmission of at least a portion of theincident radiation on the cap onto the reference sensor element.

Additional aspects are set forth in the claims.

These and other features of the invention will now be understood withreference to exemplary embodiments which are described with reference tofigures which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a cross section through an illustrative embodiment of a sensorfor practicing the present invention.

FIG. 2 is a perspective view from above of the sensor of FIG. 1.

FIG. 3 is an example of a methodology that may be employed for formingthe sensor of FIG. 1.

FIG. 4A is an example of a first pattern that may be used to define anoptical element in accordance with the teachings of the presentinvention.

FIG. 4B is an example of a second pattern that may be used to define anoptical element in accordance with the teachings of the presentinvention.

FIG. 4C is an example of a third pattern that may be used to define anoptical element in accordance with the teachings of the presentinvention.

FIG. 5 is a plan schematic showing an example of a sensor includingmultiple sensor elements in accordance with an illustrative embodimentof the invention.

FIG. 6 is an example of a pattern that may be used to define an opticalelement suitable for use with multiple sensor elements in FIG. 5 inaccordance with the teachings of the present invention.

FIG. 7 is a sectional view of a compound sensor in accordance with theteachings of the invention.

FIG. 8 shows a further embodiment where the sensor includes a referenceelement.

FIG. 9 shows a modification to the arrangement of FIG. 8.

FIG. 10 shows an exemplary embodiment of a sensor configuration that maybe used within the context of the present invention.

FIG. 11 shows in the context of a further embodiment the provision ofsensor elements on a thermally insulated table.

FIG. 12 shows in the context of a further embodiment the formation of athermal barrier between a sensor and surrounding elements on thesubstrate, 12 a being a section view and 12 b a plan view of thearrangement. FIG. 12 c shows a modification to the arrangement of FIGS.12 a & 12 b.

FIG. 13 shows a further modification to the arrangement of FIG. 12.

FIG. 14 shows in the context of a further embodiment the provision ofdie temperature sensors.

FIG. 15 shows an example of an array of the die temperature sensors ofFIG. 14.

FIG. 16 shows a further example of an array of the die temperaturesensors.

FIG. 17 shows a further example of an array of the die temperaturesensors.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to exemplaryembodiments of FIGS. 1 to 17. Although the invention has application inany electro-magnetic (EM) radiation sensing environment, for the ease ofexplanation it will now be described with reference to a preferredillustrative embodiment, that of a silicon wafer-based thermal radiationsensor. While it is possible for each of the embodiments illustratedhereinafter to be used in combination with one another it will beunderstood that the invention is not to be construed in this limitingfashion as features and components of one embodiment may or may not beused with those of another embodiment. In this way the invention is onlyto be limited insofar as deemed necessary in the light of the appendedclaims.

Electromagnetic radiation sensors often contain delicate sensingmembranes. The fragile nature of the membrane necessitates careful (withresultant cost repercussions) handling of the sensor after the membranehas been manufactured to prevent damage and yield loss. In addition, formembrane-based thermal radiation sensors, it is an advantage to packagethe sensor in a vacuum or other low pressure environment to eliminateheat loss from the absorbing membrane through gas convection andconduction. Finally, while many single point IR sensors do not use afocusing lens at all, it is an advantage in single point thermal sensorsto be able to focus the incoming radiation onto a single sensitive pointon the membrane to effectively amplify the signal. In the cases wheresingle point IR sensors are using a lens, they generally use arefractive lens of a material with a suitable shape and refractiveindex, for example germanium or other similar material.

For imaging a thermal scene onto a sensor array to produce an infraredpicture of the scene, the same requirements also apply with theadditional requirement that focusing the beam (i.e. with a lens) ishighly desirable to produce a focused image of the scene on the imageplane of a sensor array.

The sensor of the present invention addresses these and other challengesdescribed above by providing a device and method for capping the thermalsensor at the wafer level with a silicon cap. In accordance with thepresent invention a sensor device (or array of repeating sensor devices)is manufactured on one wafer substrate and a capping wafer ismanufactured on a separate substrate. The capping wafer is joined to thesensor wafer and bonded to it under controlled ambient conditions, thepreferred embodiment being under vacuum conditions. This bonded waferarrangement can be singulated or sawn into individual capped sensorchips for final packaging and sale. Such capping methodologies are welldescribed in US Application No. 20030075794 of Felton et al which isassigned to the Assignee of the present invention, and the contents ofwhich are incorporated herein by reference.

FIG. 1 shows in cross section, such a sensor device 100. The deviceincludes a sensing element 105 formed in a first silicon wafer 110 orwhat is sometimes called a sensor die. A cap 115 consisting of a siliconlid into which patterns 120 are etched to form an individual diffractingoptical element is also provided. Two possible approaches toimplementing this diffractive optical element (DOE) are known asamplitude modulation and phase modulation respectively. In the case ofamplitude modulation, the surface pattern consists of areas that allowtransmission of the radiation and areas that block the radiation. In thecase of phase modulation the pattern consists of height variations onthe surface that effectively modify the relative phase of the radiationas a function of the relative height differences of the pattern. In thisillustrated embodiment the pattern is provided on an interior surface135 of the cap, but it will be appreciated that it could also beprovided on an exterior surface 140. It will also be appreciated thatthe pattern, whose geometry is exaggerated for ease of viewing, includesa plurality of ridges 150 whose distance apart and depth is related tothe wavelength of light with which the optical element is being used.The cap is typically formed in a second silicon wafer or capping die.This pattern 120 defined in the diffracting optical element cap 115 iscapable of focusing incident radiation 125 of a given frequency onto aspecific plane of the sensor or onto a specific point on the sensor orof focusing different frequencies onto different points. The cap 115 isbonded to the first wafer using a bond or seal material 130 and thebonding defines a sealed cavity 145, which can be at a differentpressure than ambient pressure, typically a lower pressure.Alternatively the sealed nature of this cavity and the manufacturingprocess allows the ambient gas within the cavity to be different to air,for example we could use Xenon which has a lower thermal conductivitythan air or some other gas. Although a silicon cap is substantiallyopaque to incident light in the visible spectrum and therefore it may beconsidered that it occludes the light from impinging on the sensingelement within, it will be appreciated that silicon allows atransmission of light in the infra-red frequencies of the EM spectrumand therefore for this application, the provision of an IR sensor, it isa suitable material. FIG. 2 shows an example of an assembled sensordevice from which it will be seen that the sensing element is covered bythe cap provided above it.

A typical process flow for manufacture of the sensor is shown in FIG. 3.Firstly, the sensor wafer 110 is manufactured using techniques that willbe well known to those in the art (Step 300). The capping wafer is alsomanufactured (Step 310) separately. The manufacture of this cappingwafer includes the etching of a desired pattern on either or both of theouter 140 or inner surface 135 of the cap. An anti-reflective coatingmay additionally be added to the cap surface, either inner or outer.Once the desired components on each of the two wafer substrates areprovided, the wafers may be brought together so as to be bonded (Step320). Ideally, this bonding is achieved under vacuum conditions. Oncethe two wafers have been brought together individual chips may besingulated or defined within the total area of the wafers by removingthe areas of the second wafer that do not define the cap (Step 330). Inthis manner a plurality of individual chips or sensors may be providedin one process flow.

It will be understood that the nature of the pattern defining theoptical element will effect how the sensor performs. FIG. 4 showsexamples of pattern types, which can be implemented using either anamplitude modulation or a phase modulation approach, which may be usedto define diffractive optics in the sensor cap. The example of FIG. 4Ais optimised for a focusing of parallel input light of wavelength 10micrometer down to a focal plane 300 micrometer away using a sinusoidalvariation in the height of the diffractive optical element for a phasemodulation approach. The relative heights of the sinusoid arerepresented by the gray scale variation in the pattern, for an amplitudemodulation approach the gray scale would represent the transmissionefficiency of the pattern. The example of FIG. 4B is designed for afocusing of parallel input light of wavelength 10 micrometer down to afocal plane 370 micrometer away but in this case the black and whitepattern represents a single step height variation to implement thegrating of the phase modulated diffractive optical element rather than asinusoidal variation. The example in FIG. 4C also uses a single stepheight variation to implement the diffractive optical element but inthis case it is designed to focus parallel input light of wavelength 10μm down to a focal plane 10 micrometer away. It will be understood thatthese three examples are illustrative of the type of pattern that may beused and that different design requirements regarding the control of thefocus plane or independent control over different wavelength componentswithin the incident radiation are also possible with this approach andare covered by this invention. These examples, consisting of black andwhite circles in FIGS. 4B and 4C can represent either a transmissionpattern or a phase modulation pattern that focuses the light, but sufferin that losses in transmission are also achieved. It will be appreciatedhowever that the design of the pattern may be optimised to achieve lowerloss criteria such as for example introducing curved side walls in theridge features defining the grating, as represented by the grayscalediagram of FIG. 4A.

The cap provided by the present invention is advantageous in a number ofaspects. It serves to: 1) protect the membrane during subsequenthandling, 2) it also provides a housing for the sensing membrane thatcan be evacuated during manufacture, and 3) it can be patterned andetched in such a way as to focus the incident infra red radiation onto asingle point to amplify the signal or onto an array to create an imageof a scene. In particular, the pattern can be such as to implement anoptical element (i.e. conventional refractive or Fresnel lens) or in thepreferred embodiment a diffractive optical element. The creation of anoptical element for this application is advantageous in that the lenscan be implemented in silicon rather than the more exotic (andexpensive) materials required heretofore for an infrared refractivelens. The advantage resulting from the use of diffractive optics in thesilicon cap is that the lenses can be patterned and etched at the waferbatch level using well established processes and bonded to the sensorwafers, resulting in a cost effective lens compared to the refractivelens technologies heretofore employed. This approach may be applicableto other electromagnetic radiation sensors in addition to the infraredapplication described here. For example the cap could be made of quartzor in some cases standard glasses such as pyrex or possibly sapphire ifthe sensor is to be used for applications other than IR sensors.

In some applications it may also be useful to be able to use thelens/cap configuration to focus different wavelengths within theincoming radiation onto different sensors enclosed by the cap. FIG. 5 isa schematic illustration of one such example where four sensing elements501, 502, 503, 504 are provided within the same cap arrangement. It willbe appreciated that suitable designing of the lens arrangement may allowfor an optimisation of the sensor to focus one particular wavelengthwhile defocusing (rejecting) others. This would allow individualintensity measurement of different wavelength components within theinfrared radiation, a capability that could be very useful in forexample gas analysis such as alcohol breath samplers where there is adesire to monitor the level of ethyl alcohol in the breath of a person.As alcohol has specific absorbance peaks in the IR spectrum, thefocusing of radiation coincident with these peaks onto specific ones ofthe sensors elements 501, 502, 503, 504 provided in an array below thecap will enable the discrimination of any change in the intensity of theradiation at those specific frequencies therefore serve as an indicatorof alcohol present in a sample. As each of the sensor elements areconfigured to react to incident radiation of a suitable frequency, whenthat radiation is incident on the individual sensors, an analysis of theperformance of each of the sensor elements indicates the presence orabsence of the material for which it is designed to react to- providinga gas wavelength signature of the gas being analysed.

FIG. 6 is an example of a diffractive optical element (DOE) design usingan amplitude modulation approach that could be used in combination withthe sensor arrangement in FIG. 5 to focus each one of four distinctwavelengths within the incident radiation onto one of the four sensingelements 501, 502, 503, 504 that are shown in FIG. 5. Such a design orpattern could be fabricated by creating a single step in the lens orproviding multiple steps of different heights. It will be appreciatedthat the invention is not intended to be limited in any way as to thefabrication of a DOE in that it is intended to encompass all methods ofmanufacture be they single step, multiple step or other variants.

Although not shown, it will be appreciated that the structure of thepresent invention may be further modified to include a second lensarrangement provided over the optical element so as to effect a compoundlens effect. Such an arrangement may be suitable for applications suchas increasing magnification, increasing the field of view, increasedresolution and improved optical filtering. Such an arrangement could beprovided by providing a second lens coupled to the chip. Alternatively,and as shown in FIG. 7, it is possible to fabricate a second lens 701and couple that second lens to the completed package 700. In this way adefined volume 703 is created between the DOE 115 and the interiorportion of the second lens 701. The atmosphere in the defined volume maybe controlled as desired vis a vis pressure or content. It will beappreciated that any other method of fabricating a compound lens effectis intended to be encompassed within the scope of the invention.

It will be understood that the techniques of the present inventionprovide an efficient way to provide an IR sensor array such as forexample a 60×60 array. Such configurations are desirable forapplications such as IR imaging where a sensor array of the presentinvention may be used to replace conventional IR arrays. Current IRarrays do not have the lens and sensor array integrated in a low costunit as provided for by this invention. Current conventional IR arraysprovide a vacuum package with an IR transparent window or lens in thepackage rather than the wafer level solution described by thisinvention.

Another application for the integrated senor element/lens capconfiguration of the present invention is where depth of field analysisis required. By configuring the lens suitably, it is possible to focuslight from two different distances onto separate sensor elements withinthe cap. This enables discrimination as to the origin of the heatsource, for example is it a planar metal plate or a 3-Dimensional humantorso. Such applications may include discriminatory deployment sensorsfor use in for example air bag deployment arrangements.

The dimensions of a sensor in accordance with the present invention aretypically of the order of micro to millimeters. For example whentargeting radiation of a wavelength of 10 micrometers, a cap may bedimensioned to have a collection area of about 1 mm² and be of a heightof about 160 micrometers above the sensor element. These dimensions arehowever purely for illustrative purposes only and it is not intended tolimit the present invention to any one set of dimension criteria.

The fabrication of the sensor of the present invention has beendescribed with reference to an etch process. Typically this etch will beof the type of process known as deep reactive ion etching (RIE) whichinherently produces substantially vertical sidewalls (approximately 90degrees). One of the advantages of such a process is that with suchverticality less space is required for the cavity sidewalls. Thisdirectly affects the size of the “window” and thus the overall size ofthe cap which can be made. By reducing the cap size there is a reductionin the area required on the chip—with a corresponding reduction in the“wasted” space under and around the cap edges.

Cap Arrangement Incorporating a Radiation Barrier

Heretofore, a sensor in accordance with the teaching of the inventionhas been described with reference to a sensing device with a transparentwindow. The invention also provides in certain embodiments for thefabrication of a second cell also incorporating a sensing device, whichprovides a different response to that of the first cell. This secondcell then may be considered a reference cell, which differs from thefirst sensing cell in that its response may be used in combination withthe sensing cell to allow for a discrimination in the response of thesensing cell. One example of this is to make the reference cell totallyopaque so its sensor sees only the cap (i.e. 300K) in the case of IRsensors, but one could make the reference partially opaque so there wasalways a known fraction of the ambient radiation getting through. Therewould be advantages to this in applications for gas sensors where thereference cell could be illuminated with radiation coming through thesame optical path as the sensing side except for the gas to be sensed.This would remove spurious dependencies of the signal on e.g. watervapour. A further example would be where the optical characteristics ofthe second cell are the same as that of the first cell but it isselectively illuminated with radiation of a different frequency, i.e. adifferent source of radiation, so as to provide an output which isdifferent to but which can be compared with that of the first cell. Inall cases however it will be understood that the second cell isconfigured to provide a different response output to that of the firstcell with the variance in response of this second reference cell may beprovided by altering the characteristics of the cap used for the secondcell being used to reference or calibrate the output of the first cell.

Typical embodiments will employ a reference cell with an opticallyopaque window. Such opacity may be used to provide a “dark” cell, onewhich will provide a signal output that is independent of the level ofradiation being sensed by the first cell. FIG. 8 shows an example ofsuch an arrangement. The same reference numerals will be used forcomponents already described with reference to prior Figures.

In this arrangement a sensor device 800 includes a first cell 810 whichprovides an output indicative of the level of radiation incident on thesensor device and a second cell 820 which provides an output which isindependent of the level of radiation incident on the sensor device. Thefirst and second cells each include an IR sensor 105 formed on a firstsubstrate 110 and each have a cap 816, 826 provided thereabove: Thecapping of each cell serves to define a controlled volume above eachsensor, which as described above can be suitably evacuated or filledwith a specific gas depending on the application. The second cell 820differs from the first in that it is configured so as to prevent thetransmission of radiation through the cap and onto the sensor 105. Thismay be achieved by providing an optically opaque layer 830 on the cell.The second cell can therefore be considered a reference cell, whoseoutput is independent of the incident radiation. The output of thissecond cell can then be used to calibrate the output of the first cell,whose signal output will be determined by the intensity of the incidentradiation thereon.

It will be understood that by providing such a reference cell, that asensor device in accordance with the teaching of the invention enables adetection of radiation by providing for a comparison between outputs ofan exposed sensor and those of a reference darkened sensor. In thisdevice only the optical properties of the darkened sensor are changed,the thermal and electrical properties are the same as those of theilluminated sensor. In this way an accurate and precision sensing ofincoming radiation is possible—be that IR radiation or any other type ofelectromagnetic radiation such as that in the visible spectrum.

The arrangement of the two cells shown in FIG. 8 is of two distinctcells, each being formed separately. Alternative arrangements such asthat shown in FIG. 9, may provide a single cap 900 which ismicro-machined to define two cavities or chambers 905, 910, one of which905 is locatable over the illuminated element and the second 910 overthe non-illuminated element. Each of the two defined areas has an IRsensitive element 105 and may be formed using any convenient process.The interior of the cap cavities may be filled with any desirable gasambient (e.g. Air, Nitrogen, Argon, Xenon) or indeed simply provided asa vacuum. The cap is sealed to the substrate using a sealing processwhich can provide the necessary level of hermetic seal. Such techniqueswill be apparent to the person skilled in the art. The shield 830 whichblocks the IR radiation is fabricated using (conveniently) a thin metallayer which will reflect incoming radiation. In order to avoid heatingthe cap non-uniformly, desirably the IR blocking layer should be areflector, not an absorber. As shown in FIG. 9, a gap 920 in the sealingmay be left between the individual lid chambers to allow the pressure ineach chamber to equalise, independent of leak rate of the overall cap.Such an arrangement addresses the issue with many MEMS based IR sensordevices which are very sensitive to the ambient pressure.

In order to define the two chambers, a column 925 is provided. Thecolumn extends downwardly from the top 930 of the cap 900, andterminates at the gap 920 between the two chambers. The column may becoated with or doped to minimize the leakage of radiation between thetwo columns. Typical dimensions for the column are 50-100 microns wideand 170 microns high. The gap is typically of the order of 6 micronshigh which is of the order of the wavelength of the IR radiation beingmonitored so it is unlikely that any radiation could transfer throughthe gap from the illuminated cavity to the non-illuminated. However, ifrequired further guarantees of the integrity of the dark cavity could beachieved by providing a step pattern—similar to a saw tootharrangement—so as to allow the equalisation of pressure but occlude thetransfer of radiation.

To further reduce the level of IR contamination within theun-illuminated cavity side, the walls of the separation region may alsobe coated with a reflecting metal (or other IR type barrier) to block IRwhich has been reflected from the illuminated surface. Alternativelythis region may be treated (e.g. heavily doped to sufficient densityusing for example a polysilicon material or oxidized to sufficientthickness) in such a way as to absorb any reflected IR. The absorbing ofthe radiation is a preferred way to achieve the blocking of IR throughthe internal portions of the cavity as it ensures that it is taken outof the cavity as opposed to just bounced to another region—which wouldbe the case in a reflective solution. The absorption provided by theside walls serves to damp down reflections to prevent the creation ofspurious signals within each cell A further suitable technique could beto simply space the non-illumination sensor sufficiently from theillumination sensor so that the radiation will be absorbed naturally inthe silicon.

It will be understood that a sensor arrangement in accordance with theteaching of the invention provides for the use of high thermalconductivity materials for the cap so as to ensure that the two sensingdevices are exposed to the same temperature surface, thus againminimizing thermal contamination problems. While described withreference to silicon it will be understood that other materials such asgermanium could also be used.

By using a capping arrangement such as that described herein it ispossible to locate the illuminated and non-illuminated sensors adjacentto one another. As a result they can be fabricated at the samefabrication efficiency and the only difference between the two is theoptical environment in which they operate. This is particularly usefulfor sensors that are used in high sensitivity applications where lowdifferences in output between the two sensors (the reference and theactive) are indicative of an actual measurement.

By providing at least two cells which differ in their responsecharacteristics it is possible to define such active and reference cellsas has been just described. The provision of the differing responsecharacteristics can be implemented in any one of a number of differentmanners, for example by modifying the optical response characteristics,the electrical characteristics, the thermal response characteristics oreven by keeping all these three characteristics the same and justilluminating each cell with a different source of irradiation.

Use of a Wheatstone Bridge Arrangement

While the specifics of the IR sensor (e.g. bolometer, thermopile orother) are relatively unimportant within the context of shielding, FIG.9 shows an arrangement of the IR sensor in a Wheatstone bridgeconfiguration. To function, one side of the Wheatstone bridge isrequired to be illuminated while the other side is in the dark. Usingthe structure of the cap arrangement previously described it is possibleto shield the dark side of the bridge while maintaining otherwiseidentical thermal and electrical performance of the sensing elements.Such an integrated IR sensor structure combines a highly effectivethermal management scheme, a vacuum or controlled ambient cap and amethod of providing shielding for the dark side of the bridge. The capstructure ensures that the thermal and electrical properties of theshielded and illuminated bridge elements are identical. While describedin FIG. 9 as a single device, it will be understood that such anarrangement can be applied to single sensors or arrays.

In a Wheatstone bridge configuration such as that shown in FIG. 10, aheat sensitive resistor (such as a thermistor or bolometer) on one sideof the bridge (Rbol′) is illuminated by the incoming radiation and theoutput of this side of the bridge is effectively compared to itsun-illuminated pair (Rbol) to create an output voltage which isproportional to the difference in the resistance change betweenilluminated and unilluminated resistors:Vo=VDD[(Rbol−Rbol′)/(Rbol+Rbol′)]And for Rbol′=Rbol+dRdVo˜−2dR/4Rbol

The heat sensitive resistors are characterized by having a knowntemperature coefficient of resistance (TCR), and will absorb heat fromthe incoming radiation if they are illuminated. Thus it is clear thatnot alone must the resistors (Rbol) be maintained in the dark, they mustalso see the same thermal environment as Rbol′ so that no othertemperature effects are allowed to contaminate the observed signal.While other configurations of the bridge are possible and sometimesdesirable, making the bridge from 4 identical resistors (same TCR, samethermal conductance and capacitance) in which 2 of the 4 are shieldedfrom incoming radiation while otherwise maintaining the identicalthermal environment for each of the illuminated resistors, gives optimumperformance. Using identical resistors has the effect that in theabsence of any incoming radiation the output voltage will remain zerofor any change in the background temperature of the resistors. Theresistors which are not responsive to incoming radiation are oftenreferred to as ‘reference’ bolometers.

While a radiation sensor using a Wheatstone bridge configuration usingfour physically separate resistors may provide suitable signal responsesfor certain applications, it is possible to improve the performance ofthe classic bridge configuration. An embodiment of the inventionprovides such an arrangement which improves the responsiveness of thesensor to an applied signal. In this arrangement each pair of the tworesistors that form the opposing legs of the Wheatstone bridge areco-located on a thermally isolated table so as to ensure that they eachare exposed to the same thermal environment.

As shown in the plan (FIG. 11 a) and section (FIG. 11 b) view of FIG. 11(which shows the construction of a single table with two resistors) suchan arrangement provides a thermally isolated table 1100 which is formedby etching a cavity 1105 in the silicon substrate 110. The extent of thecavity may be defined by the use of trenches 1110 which can control theextent of the etch process. The cavity serves to insulate the table 1100from the substrate below. Slots 1115 can be provided to insulate thetable from any thermal gradient in portions of the chip beside thetable. First 1120 and second 1125 resistors are provided on the table,in this illustrated exemplary embodiment in a snake (S) configuration.It will be appreciated that the actual configuration of the resistors isnot important, what is important is that the main portion of thefabricated resistor is provided on a thermally insulated table. Each ofthe two resistors are provided with contact points 1130, to facilitateconnection of the resistors to the remaining portions of the bridge.

It will be understood that in this embodiment while FIG. 11 shows theformation of a table with two bridge resistors, i.e. one pairing of theWheatstone bridge, that desirably each of the pairings of the bridgeresistors are located on their own table. In this way the formation ofthe bridge configuration will require two thermally isolated tables,each of which are desirably formed using MEMS fabrication techniques.The two resistors on opposite legs of the Wheatstone bridge areco-located on the same table so as to ensure they both see the sametemperature change and, if appropriately connected, provide twice theoutput signal for a given input radiation flux density.

The heat sensitive resistors are characterized by having a knowntemperature coefficient of resistance (TCR), and will absorb heat fromthe incoming radiation if they are illuminated by it (a suitableabsorbing layer is included in the construction of the resistors andtables). A variety of absorbing layers may be used including layers ofsilicon nitride, silicon dioxide, silver compounds and resistivecompounds such as Titanium nitride, such as are well known in thisfield. The challenge is to build the resistors in such a way that theabsorbed energy creates a sufficiently large temperature rise and thento maximize the available output signal for the given temperature rise.

There are many well known advantages of a Wheatstone bridgeconfiguration. A major benefit is that for a well matched set ofresistors (e.g. all resistors having the same TCR and value) the outputvoltage is independent of the ambient temperature and only dependent onthe total voltage across the bridge (VDD) and the localized heating ofeach illuminated resistor Rbol′. In this embodiment, the maximumpossible signal is achieved by locating the two Rbol′ units on one tableand the two Rbol units on a similar table. Their construction ensuresthat the two tables are poorly coupled thermally while ensuring theradiation sensitive resistor pairs are isothermal. Although the thermalisolation of the table is slightly degraded as for high performancedevices the thermal conductance of the table is dominated by the aspectratio of the table legs. Thus, widening the legs to accommodate tworesistors will cause a decrease in the achievable thermal resistancefrom the table to the substrate (the main heat sink in such systems). Itwill therefore be understood that as the legs affect the total DCresponse and the time constant of response of the sensor, that there isa certain trade-off possible where the designer of the system may choosedifferent dimensions of legs depending on the speed of response versusaccuracy required for the system

While the above is described in terms of a Wheatstone bridge, it is notessential that such a configuration be used. For example if the sensingresistors were to be biased by an opposing pair of current sources thesame structure could be used and would produce the same benefits. Otherapplications that would benefit from the provision of the thermallyequivalent environment generated by the location of two resistiveelements on the same thermally isolated table would include the scenariowhere a resistor was used in a feedback configuration with a secondresistor providing a sensing element—both resistors combining to providethe response of the circuit and it being important that temperaturevariances between the two resistive elements did not introduce spuriousresults.

In FIG. 11 a table with 2 legs is shown but any number of legs can beused though generally there is a trade-off between mechanical stabilityand degree of thermal isolation of the table. A symmetric arrangement oflegs is desirable for mechanical stability but is not essential.

As one side of the bridge is required to be illuminated by the incomingradiation and the other to be shielded from the radiation there will beseparation between them, enough to allow the construction of a suitableshielding structure. Such a structure could be provided by the sensorarrangement of FIG. 9 where each of the pairings is provided on athermally insulated table within a specific cavity. The IR sensors 105can be provided as a resistor bridge arrangement by fabricating thebridge using silicon micromachining technology which is sensitive toinfrared radiation. The micromachining of the two cavities 905, 910 canbe effected with one of the cavities located over the illuminatedelement (Rbol′) and the other over the unilluminated element (Rbol). Asshown in FIG. 9, the unilluminated table may have a cavity 1105underneath to give best resistor electrical and thermal matching to theilluminated side (if the incoming radiation is “sufficiently” blockedfrom reaching Rbol, or it may have no cavity underneath to provide abetter thermal short circuit (thus minimising its IR response) to thesubstrate if the incoming radiation is only partially blocked.

While this embodiment has been described with reference to a preferredimplementation where two resistive elements are provided on a thermallyisolated table it will be understood that this illustration is exemplaryof the type of benefit that may be achieved using the teaching of theinvention. Such teaching may be considered as providing at least twothermally sensitive electrical elements on a first region which isthermally isolated from the remainder of the substrate. Such thermalisolation has been described with reference to the embodiment where thetable is fabricated in the substrate, but it will be understood thatequivalently a table could be fabricated on a substrate. Such astructure could be provided by for example, depositing a sacrificiallayer on an upper surface of the substrate, then the sensor elementlayers, including support layers, and then removing the sacrificiallayer, leaving a freestanding table. Alternative implementations whereinstead of the sacrificial layer, a deposited layer is provided havinghigh thermal coefficients such that it serves to thermally isolate theformed sensor elements located thereabove from thermal effects presentin the substrate. These and other modifications will be apparent to theperson skilled in the art as a means to provide a thermal barrier underthe electrical elements where high degrees of thermal isolation arerequired.

Thermal Barrier for Thermally Isolating Portions of the Die

As will be understood from the preceding, thermal sensors and otherelectrical elements can be effected by the temperature of the supportingsubstrate. It will be understood that thermal sensors specifically aresensitive by design to changes in temperature and often use thetemperature of the supporting substrate as a reference or baselinetemperature. However, if the sensor incorporates heat generating means(e.g. circuits) which are nearby then this reference or baselinetemperature will be disturbed and will give rise to an error in thecalculated temperature measured by the sensor.

FIG. 12 shows another embodiment which illustrates a means of creating athermal barrier between any heat generating region and the thermalsensor to isolate the sensor from any spurious source of heat. Thedegree of thermal isolation required can be engineered as needed. Thisembodiment uses pairs (or more, e.g. triples, etc.) of trenches spacedby gas or vacuum containing regions coupled possibly withsilicon-on-insulator wafers to provide a high degree of thermalisolation.

While important for all such sensors and circuits the problem becomesparticularly acute if the sensor is attempting to measure a thermalsignal itself, e.g. an infrared sensor or a micro-calorimetry sensor,and therefore is particularly suited for incorporation in the sensorarrangements described with reference to FIGS. 1 to 11. Such sensorsgenerally take the substrate temperature as a reference or baselinetemperature and perform a comparison of their internal temperature withthe substrate reference or baseline temperature. Any offset or unequaldistribution in the baseline temperature, whether steady state or timevarying can lead to inaccuracies in the sensor response in essentiallydirect proportion to the experienced offset.

Any nearby circuitry which is dissipating heat will cause a localtemperature rise around the power dissipating element. This heat isconducted away from the element in a manner which is well controlled andunderstood according to the thermal conductivity properties of thematerials used. While the thermal barrier may be suited for applicationwith the capped sensors heretofore described, it will be now illustratedwithout reference to such capping.

This embodiment of the invention provides for a means for increasing thethermal resistance between a heat source and a sensor or sensitivecircuit to reduce the impact of this extraneous heat source. Withreference to FIG. 12, the main features of the structure and a method ofmaking it are described. The same reference numerals will be used forparts previously described with reference to any one of FIGS. 1 to 11. Asilicon or other suitable wafer is used as the substrate 110. The sensoris formed by any suitable method (the order of fabrication is notimportant) and then two adjacent deep trenches 1205, 1210 are etched inthe substrate so as to form a ring around either the sensor 105 or theheat dissipating source 1215 and filled with an appropriate fillingmaterial 1205 a, 1205 b (e.g. silicon dioxide, silicon nitride,polysilicon or any combination of these). Each of the two trenchesthereby form a first and second region having a first thermalcoefficient and are separated from one another by an intermediary region1220 which has a second thermal coefficient. This intermediary regionmay conveniently be provided by etching a region between the twotrenches to form an air filled region which may be capped by means of aninsulating cap 1225 to form an evacuated air filled region. Of coursethe available volume within this region 1220 could alternatively befilled with other gaseous compositions, as required. The remainder ofthe circuit elements are formed by any of the usual integrated circuitmethods leaving the insulating cap 1225 over the trenches and the regionbetween them. Occasional holes 1230 for etchant ingress are formed inthis insulating layer exposing the silicon between the adjacenttrenches. The entire wafer is then exposed to an etching medium, (e.g.XeF2 if a gas or KOH if a liquid etchant), chosen so as to etch only thesilicon, which removes the silicon between the trenches. The low thermalconductivity of this gas filled space ensures that little heat istransferred from source to sensitive element down to the depth of thetrenches and removed zone. The heat is forced to flow under the trenchedand etched area thus increasing the effective thermal resistance betweenthe heat source and the sensitive element.

Increased levels of thermal isolation can be obtained in a number ofdifferent manners such as by (1) using multiple such thermal barriers,(2) increasing the depth of the trenches and removed zone relative tothe overall depth of the silicon substrate, (3) as shown in FIG. 12 c,by extending the etch sufficiently that an undercutting 1240 under thesensor element is defined so creating to a greater or lesser extent anenclosed isolated “island” 1250 or (4) by using a substrate ofsilicon-on-insulator material 1301 and ensuring that the trenches areetched to the depth of the buried insulator layer as shown in FIG. 13.The person skilled in the art will be familiar in the fabrication ofstructures incorporating such buried oxide layers. As the oxide or otherdielectric layers have much lower thermal conductivity thansemiconductor or metallic layers, the use of such materials mayadditionally improve the thermal isolation achieved. The extent of theetch shown in FIG. 12 c can be controlled by having one of the trenchesformed deeper than the other, typically the outer trench, so that theetch is achieved preferentially in a direction under the sensor. Theprovision of this evacuated region under the sensor creates a region ofdifferent thermal characteristics to that of the surrounding siliconsubstrate and thereby provides increased thermal insulation for thesensor located above.

Advantageously the trenches could be filled with a dielectric such assilicon dioxide provided using any known method. However, because of thedifference in coefficients of thermal expansion between silicon dioxideand silicon it has been more common to use a thin layer (e.g. 100-200nm) of silicon dioxide or silicon nitride or both to line the etchedtrenches and then fill the bulk of the trench with polysilicon. Thissignificantly decreases the effectiveness of the trench as a thermalbarrier due to the high thermal conductivity of polycrystalline silicon.In principle the trenches could be left unfilled after the silicondioxide layer is deposited but this gives rise to problems of surfacepotential control at the trench/silicon substrate interface which isgenerally undesirable if the trench has any function other than thermalisolation. Also, as it is desired to ensure cost is kept to a minimum,our approach uses steps which are in many situations used for thefabrication of the sensor itself and thus add no further cost to thefabrication process. Due to the surface potential control problemmentioned above, this would probably not be acceptable in the case foran unfilled trench (i.e. a trench used in the process for electricalisolation could not tolerate such a surface potential control issue). Itis also frequently desired to perform the trench processing at thebeginning of the fabrication sequence for the whole process and anunfilled trench could not be tolerated in this case as it would fillwith process residues leading to unmanageable defect levels due to itsopen top. The method we suggest is intended to be carried out at the endof the process sequence, using trenches which have been fabricated atany point in the process, thus avoiding any such issues.

The formation of a thermal barrier, by defining regions of differentthermal coefficients, around the temperature sensitive element serves toisolate the element from the effects of any heating from adjacentcomponents. It will be appreciated that it may still be necessary toelectrically couple components within the thermal barrier region tothose outside the region. Such coupling may be achieved by providing anyone of a number of different types of electrical connection, such as awire track 1235, between the components that need to be coupled.Depending on the circumstances of application of such sensors differentdegrees of thermal isolation may be required which will affect theultimate thermal barrier configuration chosen.

Distributed Die Temperature Sensor

While the sensors heretofore described have been described withreference to stand-alone sensors or arrays of such sensors, in anotherembodiment of the invention an arrangement which provides for dietemperature sensing is also provided. Such an arrangement is shown inFIGS. 14 to 17.

The provision of such die sensing provides a means of measuring the dietemperature at a number of locations around the die, these temperaturemeasurements can then be used to re-compensate the apparent observedtemperature. This may be done by locating small temperature measurementmeans, such as sense diodes and/or transistors at strategic pointsaround the die and using temperature sensor circuits to measure thesespot temperatures, providing the data to the user. Where used incombination with the thermal barrier that was illustrated above withreference to FIGS. 12 & 13, the efficacy of the thermal barrier can alsobe checked in production by applying a known heat source and measuringthe change in temperature across the barrier, dT, using sensors locatedon either side of the thermal barrier.

It is known that any circuitry which is dissipating heat will cause alocal temperature rise around a power dissipating element. This heat isconducted away from the element in a manner which is well controlled andunderstood according to the thermal conductivity properties of thematerials used. In addition, other die temperature changes at local orwider levels can be caused by external sources of radiation or ambienttemperature changes. For example, if an IR thermal sensor is measuring ascene which includes a moving hot-cold edge (e.g. a hot object on aconveyor), a die temperature change will occur on the sensor die, movingfrom one edge to the other as the object transits the field of view.Likewise, if the same thermal sensor is carried from a cold environmentto a hot one, the base die temperature will change with some appreciabletime lag causing reading inaccuracies. Thus, both global die temperaturechanges and fixed or time varying temperature gradients can cause severemeasurement inaccuracies.

Another problem that occurs for MEMS implementations of the thermalsensor, where a thermal barrier has been etched into the siliconsubstrate, is that the thermal barrier may be breached or incompletelyformed, either at the silicon processing stage or during the cappingprocess that was described with reference to previous figures. Somemeans of checking this barrier at probe and final test is useful toeliminate poorly performing die.

The problem can be significantly better managed if a number oftemperature sensors are located around the die area and the localisedtemperature readings are then used to compensate the IR thermal sensormeasurements. The application is particularly important for thermalsensor arrays used for radiometric applications (i.e. where actualtemperature measurement is needed as opposed to thermal imaging), wheredie temperature gradients can cause severe temperature measurementinaccuracies.

In this embodiment of the invention, we disclose within a system such asa thermal infrared or microcalorimetric sensor or imaging system, adistributed set of temperature sensing points located within and withoutthe thermal barrier (i.e. on either side of a thermal barrier), if thatexists. These temperature sensing points can be made in many ways butadvantageously they are made using PN junctions which are then drivenwith circuits well known to those skilled in the art. FIG. 14 shows howsuch an arrangement might look. The same reference numerals are used forcomponents described with reference to previous drawings.

As shown in FIG. 14 a plurality of individual die temperature sensors1400 are provided, and are arranged about the die. In the views of FIG.14, a plurality of individual IR sensors 105, similar to those describedpreviously, are provided within a thermal barrier region 1410 defined bytrenches 1205, 1210. Die temperature sensors 1400 are provided withinthis thermal barrier region to monitor the die temperature within theregion. Additional die temperature sensors are provided outside thethermal barrier region 1410. To check the thermal barrier integrityformed by the trench arrangement, these temperature sensing devices 1400are combined with sources of known heat load such as resistors 1215. Insome embodiments the heat load function can be combined with a known TCRresistor in a single unit thus combining the functions of heating andtemperature sensing to minimise space use. Alternatively, the heatercould also simply be a known source of heating on the die e.g. a nearbyhigh current region such as the input stage of an amplifier. The arraytemperature sensors could also be used to perform the same function,i.e. for a known circuit condition where a source of heat is availableexternal to the thermal isolation barrier, the internal array of sensorswill show a specific pattern of temperature measurements. Any deviationfrom this once characterised, will point out a thermal barrier defect.

When a known heat load is applied through the resistor(s) temperaturescan be measured on either side of the barrier to ensure its integrity.These temperature differences would be characteristic of any givensystem and the package it is located within. Any defect or fault in thebarrier (e.g. bridging of the thermal isolation trench by someextraneous material) would result in a temperature difference smallerthan expected.

To assist in improving the accuracy of the thermal sensor measurement,the die temperature measurement devices 1400 need to be distributedaround the die within the thermal barrier so that local, time varyingtemperature measurements of the die next to individual sensor pixels andgradients across the die can be known. The user can then select to makeeither average die temperature measurements (average all readings)during the course of a measurement or in applications which experiencesharp thermal scene temperature differences (either spatially ortemporally) the local temperature reading can be used to improve thetemperature measurement accuracy of any individual pixel.

FIGS. 15 to 17 show a distributed temperature sensor for a small array.A variety of temperature sensor placing strategies can be used. As shownin FIG. 15, temperature sensors 1400 may be placed at all corners of thesmall 3×3 pixel array 1500. This gives detailed knowledge of the dietemperature “map” but is costly in terms of area used and wiringrequired to connect all the sensors. FIG. 16 shows an alternative where,for this size array, all outside corners and all 4 interior corners hosttemperature sensors 1400. This gives 2 sensors for each pixel and 4 forthe centre pixel, which could be advantageous in some applications wheregreatest accuracy is required of a specific pixel in the array. FIG. 17shows a minimal coverage situation where only the 4 interior cornershost temperature sensors. In this situation there is at least onetemperature sensor per pixel, some have two and the centre pixel againhas 4 sensors. This distribution may be less effective in the detectionof thermal barrier defects (depending on the size of the array and/orarea inside the thermal barrier) as the sensors are located further fromthe barriers. It will be understood from the different examples of FIGS.15 to 17 that different applications will require other distributions ofdie temperature sensors. Furthermore it will be understood that thedistribution of the individual die temperature sensors may require someof the die temperature sensors to be located under the caps of thesensor with others located outside the caps.

It will be understood that an arrangement such as that provided by thepresent invention offers many advantages over the existing state of theart. Current practice in thermal radiometric measurement systems is tomeasure the die temperature with an external temperature sensor locatedin close thermal contact or proximity with the sensor package. Thisambient temperature measurement unit is usually mounted on the same PCBor may be mounted in physical contact with the radiometric sensor orarray. Some sensors and arrays will have a sensor located physically onthe same die but never an array of die temperature sensors has beenused. If, the die temperature sensors are made using known circuittechniques for building active temperature sensors then the user willhave access to pre-calibrated die temperature information in greatdetail, not requiring him to undertake extensive calibration of thisparameter.

Sensing the die temperature with an additional sensor located outsidethe package gives rise to an apparent thermal lag between actual arraytemperature and ambient or PCB temperature. This leads to unacceptablylong periods of inaccurate readings for many applications. The schemedisclosed here provide far superior measurement of the die temperaturewithout this thermal lag.

It will be understood that the sensors described herein have beenillustrated with reference to exemplary embodiments. It will beunderstood that the features of any one embodiment may be used withthose of another embodiment or indeed can be applied independently ofthe structural features of the other embodiment. Applications for suchsensors can be in a plurality of environments such as IR to Digitalconverters, both single pixel and arrays. Further applications includesingle point thermal measurement systems, e.g., digital thermometers,intruder alarms, people counting sensors, and into infrared cameras tothermally image scenes. These and other applications will be readilyapparent to the person skilled in the art on review of the teaching setforth herebefore. Therefore while the invention has been described withreference to preferred embodiments it will be understood that it is notintended that the invention be limited in any fashion except as may bedeemed necessary in the light of the appended claims.

The words upper, lower, inner and outer are used for ease of explanationso as to illustrate an exemplary illustrative embodiment and it in notintended to limit the invention to any one orientation. Similarly, thewords comprises/comprising when used in this specification are tospecify the presence of stated features, integers, steps or componentsbut does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof. Furthermorealthough the invention has been described with reference to specificexamples it is not intended to limit the invention in any way except asmay be deemed necessary in the light of the appended claims, and manymodifications and variations to that described may be made withoutdeparting from the spirit and scope of the invention.

1. A thermal sensor having a first radiation sensing element provided ina first substrate, the first radiation sensing element providing anoutput indicative of the intensity of incident radiation on the sensingelement, the sensor further including a thermal barrier located betweenthe first radiation sensing element and a heat source co-located on thesame first substrate, the thermal barrier including at least one set oftrenches, each set having at least a first and second trench, thetrenches of each set being separated from one another by an evacuatedcavity, the thermal barrier being physically separated from each of thefirst radiation sensing element and the heat source.
 2. The sensor asclaimed in claim 1 wherein the thermal barrier is dimensioned so as toextend vertically into the substrate in a plane substantiallyperpendicular to the surface of the substrate.
 3. The sensor as claimedin claim 1 wherein the cavity is populated with a gas other than air. 4.The sensor as claimed in claim 1 wherein the cavity is dimensioned toextend below the first radiation sensing element.
 5. The sensor asclaimed in claim 4 wherein a trench is circumferentially arranged aboutthe first radiation sensing element and the cavity extends completelyunder the sensing element.
 6. The sensor as claimed in claim 1 whereinthe substrate is formed from a silicon-on-insulator wafer and thethermal barrier extends downwardly to meet the insulator portion of thewafer.
 7. The sensor as claimed in claim 1 further including a secondradiation sensing element formed in the first substrate, and a first andsecond cap formed in a second substrate, the first and second substratesbeing arranged relative to one another such that each of the first andsecond sensor elements having a respective cap provided thereabove, andwherein the cap for the first sensor element allows a transmission ofincident radiation through the cap and onto the sensor element and thecap for the second sensor element modifies a transmission of radiationthrough the cap and onto the second sensor element, the thermal barrierproviding additional thermal insulation between the second sensingelement and a heat source co-located on the first substrate.
 8. Thesensor of claim 7 wherein the cap for the first sensing element includesan optical element configured to provide a focusing of the incidentradiation onto the first sensing element.
 9. The sensor as claimed inclaim 8 wherein the at least one optical element is a diffractiveoptical element.
 10. The sensor as claimed in claim 8 wherein the atleast one optical element is a refractive optical element.
 11. Thesensor as claimed in claim 8 wherein the optical element is formed in aninner surface of the cap, adjacent to the sensing element.
 12. Thesensor as claimed in claim 8 wherein the optical element is formed in anouter surface of the cap, remote from the sensing element.
 13. Thesensor as claimed in claim 8 wherein optical elements are formed in bothan outer surface and inner surface of the cap for the first sensingelement, the combination of the optical elements forming a compoundlens.
 14. The sensor as claimed in claim 8 wherein a plurality ofsensing elements are formed and the optical element is configured toselectively guide radiation of specific wavelengths to pre-selected onesof the plurality of sensing elements.
 15. The sensor of claim 7 whereinthe cap for the second sensing element includes a reflective coatingwhich reflects radiation incident on the cap.
 16. The sensor of claim 7wherein the cap for the second sensing element includes an opticallyopaque coating so as to prevent transmission of radiation through thecap and onto the second sensing element.
 17. The sensor as claimed inclaim 7 wherein the arrangement of the first and second substratesrelative to one another define a cavity between each of the caps andtheir respective sensing elements.
 18. The sensor as claimed in claim 17wherein each of the cavities for the first and second sensing elementsare in fluid communication with one another.
 19. The sensor as claimedin claim 17 wherein each of the cavities for the first and secondsensing elements are isolated from the other of the cavities for thefirst and second sensing element.
 20. The sensor as claimed in claim 17wherein the ambient conditions and composition within the cavities canbe specified.
 21. The sensor as claimed in claim 20 wherein the cavitiesare provided at a pressure lower than ambient pressure.
 22. The sensoras claimed in claim 20 wherein the cavities are populated with a gaseouscomposition selected for the application with which the sensor is to beused.
 23. The sensor as claimed in claim 22 wherein the gaseouscomposition comprises a gas having a thermal conduction less than thethermal conduction of nitrogen.
 24. The sensor as claimed in claim 7wherein the first and second substrates are provided in silicon.
 25. Thesensor as claimed in claim 7 wherein the first and second sensingelements are infra-red sensor elements.
 26. The sensor as claimed inclaim 7 wherein the caps for the first and second element are formed inthe same second substrate, the sensor additionally comprising an outercap formed in a third substrate, the third substrate being orientatedover the second substrate, the cap formed in the third substrateincluding an optical element.
 27. The sensor as claimed in claim 26wherein the first and second sensing elements are arranged in aWheatstone bridge configuration.
 28. The sensor as claimed in claim 27wherein the Wheatstone bridge configuration is provided by the firstsensing element having a first pair of resistors and the second sensingelement having a second pair of resistors elements, resistors from eachpair defining opposite legs of the Wheatstone bridge.
 29. The sensor asclaimed in claim 28 wherein each of the resistors on opposite legs ofthe Wheatstone bridge are co-located on a thermally insulated table. 30.The sensor as claimed in claim 29 wherein the thermally insulated tableis fabricated using micro-electro-mechanical techniques.
 31. The sensoras claimed in claim 7 wherein, on arranging each of the first and secondsubstrates relative to one another, each of the caps are formed withside walls extending upwardly from the first substrate and supporting aroof therebetween, the roof being in a plane substantially parallel tothe sensing elements.
 32. The sensor as claimed in claim 31 wherein eachof the first and second sensor elements are adjacent to one another, thecaps provided thereabove sharing a common central column that extendsdownwardly from the roof, thereby defining chambers for each of thefirst and second sensing elements.
 33. The sensor as claimed in claim 32wherein the chamber for the second sensing element is treated to preventa transmission of radiation through the cap and onto the second sensingelement.
 34. The sensor as claimed in claim 33 wherein the treatmentincludes a doping of the side walls of the chamber.
 35. The sensor asclaimed in claim 33 wherein the treatment includes the application of areflective coating on the roof of the cap for the second sensingelement.
 36. The sensor as claimed in claim 32 wherein the centralcolumn does not extend fully from the roof to the first substrate, suchthat a gap is defined between a lower surface of the column and an uppersurface of the first substrate.
 37. The sensor as claimed in claim 36wherein the width of the gap is comparable with the wavelength of theincident radiation being sensed.
 38. The sensor as claimed in claim 36wherein the provision of the gap allows for an equalisation of pressurebetween the chambers for the first and second sensing elements.
 39. Thesensor as claimed in claim 7 wherein each of the first and secondsensing elements are provided as a bolometer.
 40. The sensor as claimedin claim 7 wherein at least a portion of one of the first and secondsensing elements are suspended over a cavity defined in the firstsubstrate, the cavity providing thermal insulation between the firstsubstrate and the radiation sensing element which is providedthereabove.
 41. The sensor as claimed in claim 1 wherein each of thetrenches forming the thermal barrier are filled with a thermallyinsulating material.
 42. The sensor as claimed in claim 1 furtherincluding a plurality of substrate temperature sensors, each of theplurality of the substrate temperature sensors providing an outputindicative of a temperature of the substrate on which the substratetemperature sensor is located.
 43. The sensor as claimed in claim 42wherein the plurality of substrate temperature sensors are arrangedabout the substrate, so as to give a plurality of output measurements,each of the output measurements being related to the temperature at thelocation of that substrate temperature sensor.
 44. The sensor as claimedin claim 43 wherein at least a portion of the plurality of substratesensors are located on opposite sides of the thermal barrier so as togive output measurements which are indicative of the substratetemperature on each side of the thermal barrier.
 45. A sensor arrayincluding a plurality of sensors, each of the sensors having an activesensor element and a reference sensor element, the active sensor elementbeing formed in a first substrate and having an optical element formedin a second substrate, the first and second substrates being configuredrelative to one another such that the second substrate forms a cap overthe sensor element, the optical element being configured to guideincident radiation on the cap to the sensing element, the referencesensor element also being formed in a first substrate and having a capformed in a second substrate, the first and second substrates beingconfigured relative to one another such that the cap is located over thereference sensor element, the cap serving to shield the reference sensorelement from at least a portion of the incident radiation on the cap,and wherein the substrate includes at least one additional heat source,the heat source being thermally separated from the plurality of sensorsby provision of a thermal barrier extending downwardly from an uppersurface of the substrate and into the substrate and located between theheat source and the plurality of sensors, the thermal barrier includingat least one set of trenches, each set having at least a first andsecond trench, the trenches being separated from one another by anevacuated cavity, the thermal barrier providing a discontinuity in thesubstrate material between the plurality of sensors and the heat source.46. The sensor array of claim 45 wherein an output of the sensor arraydefines an image plane.
 47. The sensor array of claim 45 wherein thethermal barrier includes at least one set of trenches, the set oftrenches having at least a first and second trench, the trenches beingseparated from one another by a cavity.
 48. A discriminatory sensorconfigured to provide a signal on sensing a heat emitting body, thesensor including a first sensor element configured to provide a signalon sensing the body a first distance from the sensor and a second sensorelement configured to provide a signal on sensing the object a seconddistance from the sensor, each of the first and second sensor elementsincluding at least one sensing element formed in a first substrate andat least one optical element formed in a second substrate, the first andsecond substrates being configured relative to one another such that thesecond substrate forms a cap over the at least one sensing element, theat least one optical element being configured to guide incidentradiation on the cap to the at least one sensor element, the sensorfurther including a thermal barrier located between the sensor elementsand substrate-provided heat sources co-located on the same firstsubstrate, the thermal barrier including at least one set of trenches,each set of trenches having at least a first and second trench, thetrenches being separated from one another by an evacuated cavity, thethermal barrier being physically separated from the at least one sensorelement and the heat sources.
 49. The discriminatory sensor of claim 48wherein the at least one sensing element of each of the first and secondsensor element are formed in the same substrate.
 50. The discriminatorysensor of claim 48 wherein the object is a human torso.
 51. A gasanalyser including at least one sensor element formed in a firstsubstrate and at least one optical element formed in a second substrate,the first and second substrates being configured relative to one anothersuch that the second substrate forms a cap over the at least one sensorelement, the at least one optical element being configured to guideincident radiation on the cap to the at least one sensor element, theincident radiation guided having a wavelength indicative of the presenceof a specific gas, the gas analyser including at least one referencesensor element formed in the first substrate and having a cap for the atleast one reference sensor element formed in a second substrate, the capserving to shield the reference sensor element from the incidentradiation on the cap such that the reference sensor element provides anoutput independent of the intensity of the incident radiation andwherein the analyser is thermally insulated from heat sources co-locatedon the first substrate by provision of a thermal barrier between theanalyser and the heat sources, the thermal barrier including at leastone pair of trenches extending vertically downwardly into the firstsubstrate from an upper surface thereof, the trenches being separatedfrom one another by an evacuated cavity, such that the thermal barrierprovides a discontinuity in the substrate material between the referencesensor element and the heat sources.
 52. The gas analyser of claim 51including a plurality of sensor elements and a plurality of associatedoptical elements therefore, each of the combined sensor elements andoptical elements being configured for specific wavelength analysis suchthat the output of the plurality of sensor elements may be used toprovide a gas wavelength signature spectrum.
 53. A method of forming asensor, the method comprising: forming at least one sensor element andat least one reference sensor element in a first substrate, forming athermal barrier around the sensor elements, the thermal barrierincluding a pair of trenches extending downwardly into the substratefrom an upper surface thereof and being separated from one another by anevacuated cavity and serving to thermally insulate the sensor elementsfrom other heat sources co-located on the first substrate by providing adiscontinuity in the substrate material between the sensor elements andthe other heat sources, forming an optical element and a shielding capin a second substrate, bonding the first and second substrates togethersuch that the second substrate provides the optical element over thesensor element, the optical element is configured to guide incidentradiation onto the sensor element and the second substrate provides theshielding cap over the reference sensor, the shielding cap serving toprevent a transmission of at least a portion of the incident radiationon the cap onto the reference sensor element.
 54. An electromagneticsensor fabricated in a semiconductor process, the sensor including firstand second sensing elements formed in a first substrate, each of thefirst and second substrates having a respective cap defined thereabove,the caps being formed in a second substrate and being mountable onto thefirst substrate and wherein the cap formed over the first sensingelement allows a transmission of radiation through the cap onto thesensing element and the cap formed over the second sensing elementfilters the transmission of radiation through the cap onto the sensingelement so as to reduce the radiation incident onto the second sensingelement relative to that incident onto the first sensing element, andwherein the sensor is provided within a thermally isolated region on thefirst substrate, the isolation being provided by provision of at leastone pair of trenches between the sensor and a heat source co-located onthe first substrate, individual trenches of the pair of trenches beingseparated from one another by an evacuated cavity.
 55. A thermal sensorhaving a first radiation sensing element provided in a first substrate,the radiation sensing element configured to provide an output indicativeof the intensity of incident radiation on the sensing element, thesensor further including a thermal barrier located between the firstradiation sensing element and a heat source co-located in the same firstsubstrate, the thermal barrier including at least one set of trenches,each set having at least a first and second trench, the trenches beingseparated from one another by an evacuated cavity providing a break inthe substrate material between the sensing element and the heat source,the thermal barrier being physically separated from each of theradiation sensing element and the heat source.
 56. A thermal sensorhaving a first radiation sensing element provided in a first substrate,the radiation sensing element configured to provide an output indicativeof the intensity of incident radiation on the sensing element, thesensor further including a thermal barrier located between the firstradiation sensing element and a heat source co-located on the same firstsubstrate, the thermal barrier including at least one set of trenches,each set having at least a first and second trench, the trenches beingseparated from one another by an evacuated cavity dimensioned to extendbelow the first radiation sensing element and wherein a trench iscircumferentially arranged about the first radiation sensing element andthe cavity extends completely under the sensing element, the thermalbarrier being physically separated from each of the radiation sensingelement and the heat source.
 57. A thermal sensor having a firstradiation sensing element provided in a first substrate, the radiationsensing element configured to provide an output indicative of theintensity of incident radiation on the sensing element, the sensorfurther including a thermal barrier located fully within the substrateand between the first radiation sensing element and a heat sourceco-located on the same first substrate, the thermal barrier including atleast one set of trenches, each set having at least a first and secondtrench, the trenches being separated from one another by an evacuatedcavity and wherein the substrate is formed from a silicon-on-insulatorwafer and the thermal barrier extends downwardly from an upper surfaceof the substrate to meet the insulator portion of the wafer, the thermalbarrier being physically separated from the first radiation sensorelement and the heat source.
 58. A thermal sensor having a firstradiation sensing element provided in a first substrate, the radiationsensing element providing an output indicative of the intensity ofincident radiation on the sensing element, the sensor further includinga thermal barrier located between the first radiation sensing elementand a heat source co-located on the same first substrate, the thermalbarrier including at least one set of trenches, each set having at leasta first and second trench, the trenches being separated from one anotherby an evacuated cavity, the sensor further including a second radiationsensing element formed in the first substrate, and a first and secondcap formed in a second substrate, the first and second substrates beingarranged relative to one another such that each of the first and secondsensor elements having a respective cap provided thereabove, and whereinthe cap for the first sensor element allows a transmission of incidentradiation through the cap and onto the sensor element and the cap forthe second sensor element modifies a transmission of radiation throughthe cap and onto the second sensor element, the thermal barrierproviding additional thermal insulation between the second sensingelement and a heat source co-located on the first substrate and whereinon arranging each of the first and second substrates relative to oneanother, each of the caps are formed with side walls extending upwardlyfrom the first substrate and supporting a roof therebetween, the roofbeing in a plane substantially parallel to the sensing elements, thecaps sharing a common central column that extends downwardly from theroof, thereby defining chambers for each of the first and second sensingelements.